1. Field of the Invention
This invention relates to a brake fluid pressure control apparatus in a skid control system for a vehicle having at least one wheel and a brake for the wheel in which the brake fluid pressure to the wheel cylinder of the brake for the wheel is controlled in accordance with the rotational condition or skid condition of the wheel, and more particularly to a brake fluid pressure control apparatus of the type in which, when the brake for the wheel is relieved, brake fluid discharged through a fluid pressure control valve device from the wheel cylinder of the brake is returned to a pressure fluid supply conduit connected to a master cylinder by a fluid pump.
2. Description of the Prior Art
Recently, various kinds of skid control systems have been developed for a vehicle having at least one wheel and a brake for the wheel, by which skid control operation for the wheel is performed to avoid a locked wheel condition and obtain good braking performance on any road. In any of the skid control systems, the rotational condition or skid condition of the wheel such as deceleration, slip and acceleration is measured by the control unit which receives the detecting signal of the wheel speed sensor which is associated with the wheel for detecting the rotational speed of the wheel. The brake fluid pressure to the brake for the wheel is controlled on the basis of the measurement of the control unit.
Methods for measuring the rotational condition or skid condition of the wheel can be roughly classified into three. In the first method, slip of the wheel is obtained from the vehicle speed and the wheel speed, and the obtained slip of the wheel is compared with a predetermined slip. In the second method, acceleration or deceleration of the wheel is obtained, and the obtained acceleration or deceleration of the wheel is compared with a predetermined acceleration or deceleration. And in the third method, slip and acceleration or deceleration of the wheel are obtained from the vehicle speed and wheel speed, and the obtained slip and acceleration or deceleration of the wheel are compared with the predetermined slip and acceleration or deceleration.
A brake fluid pressure control apparatus is arranged between a master cylinder as a brake pressure generating member and the brake for the wheel. Control signals from the control unit as the measurement result are supplied to the brake fluid pressure control apparatus for increasing and decreasing, or increasing, maintaining at constant, and decreasing the brake pressure to the brake for the wheel.
One example of the brake fluid pressure control apparatus includes a brake fluid pressure control valve device to which the control signals from the control unit are supplied, to control the brake fluid pressure to the wheel cylinder of the brake for the wheel, a hydraulic reservoir for reserving the brake fluid discharged through the brake fluid pressure control valve device from the wheel cylinder of the brake, when relieved, and a fluid pump for returning the brake fluid from the hydraulic reservoir into the pressure fluid supply conduit connecting the master cylinder and the brake fluid pressure control valve device.
In the example of the brake fluid pressure control apparatus, the brake fluid from the wheel cylinder of the brake is discharged into the hydraulic reservoir to relieve the brake for the wheel, and it is returned to the pressure fluid supply conduit by the fluid pump. When the brake for the wheel is relieved, the brake fluid pressure control valve device takes the position to reduce the brake pressure, and therefore to cut off the communication between the master cylinder and the wheel cylinder of the brake for the wheel. Accordingly, the fluid pressure of the brake fluid returned by the fluid pump is applied to the piston of the master cylinder connected to the brake pedal which is treaded by the foot of the drive. The brake pedal is pushed against the tread of the driver. Thus, whenever the brake fluid pressure to the wheel cylinder of the brake changes, the piston of the master cylinder is displaced backwards and forwards. That is called "kick-back". The driver feels disagreeable. The pedal feeling is bad.
In order to solve the above described problem, this applicant proposed the brake fluid pressure control apparatus in which a first check valve is arranged in a pressure fluid supply conduit connecting a master cylinder and a fluid pressure control valve device, being opened when the brake fluid flows from the master cylinder toward the fluid pressure control valve device, an outlet of a fluid pump is connected to the pressure fluid supply conduit between the first check valve and the fluid pressure control valve device, and a second check valve is arranged in a pressure fluid return conduit connecting the master cylinder and the wheel cylinder, being opened when the brake fluid flows from the wheel cylinder toward the master cylinder. When the brake fluid is returned to the pressure fluid supply conduit by the fluid pump, it is checked from flowing toward the master cylinder by the first check valve. Thus, the above-described "kick-back" phenomenon can be avoided. When the brake fluid pressure of the master cylinder is decreased, the brake fluid returns through the second check valve to the master cylinder from the wheel cylinder.
However, in the above-described apparatus, the brake fluid pressure in the wheel cylinder cannot be lowered under the minimum actuatable pressure of the second check valve which amounts, for example, to 0.5 to 1.0 kg/cm.sup.2. In other words, the brake fluid cannot flow toward the master cylinder under the minimum actuatable pressure. Thus, after the brake pedal is released from treading, some pressure, although it is very low, remains applied to the wheel cylinder. Particularly, when the brake is of the disc brake type, braking parts rub with each other due to the residual pressure of the wheel cylinder. The parts wear, resulting in deterioration and shortening of the life time of the brake. The vehicle running resistance becomes higher. That is uneconomical.